Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $227.67   13.116%   $257.5312   $257.53   $257.55   $257.50  
  Purchase #2 $94.32   3.447%   $97.5712   $97.57   $97.55   $97.60  
  Purchase #3 $151.86   9.702%   $166.5935   $166.59   $166.60   $166.60  
  Purchase #4 $155.38   4.439%   $162.2773   $162.28   $162.30   $162.30  
  Purchase #5 $243.98   10.381%   $269.3076   $269.31   $269.30   $269.30  
  Purchase #6 $140.95   11.449%   $157.0874   $157.09   $157.10   $157.10  
  Purchase #7 $89.90   6.701%   $95.9242   $95.92   $95.90   $95.90  
  7 purchase totals = $1,104.06       $1,206.2924   $1,206.29   $1,206.30   $1,206.30